Palladium-phosphorus/sulfur nanoparticles (NPs) decorated on graphene oxide: synthesis using the same precursor for NPs and catalytic applications in Suzuki-Miyaura coupling

Catalytic system having an organotellurium ligand on graphene oxide: immobilization of Pd(0) nanoparticles and application in heterogeneous catalysis of cross-coupling reactions

First heterogeneous catalytic system, having a covalently linked hybrid bidentate organotellurium ligand [i.e., PhTe-CH2-CH2-NH2] on the surface of graphene oxide, has been synthesized with immobilized and stabilized Pd(0) nanoparticles. To the best of our knowledge, it is the first such catalytic system in which a heterogenized organotellurium ligand has been used. It has been well-characterized using different physicochemical characterization techniques viz. P-XRD, XPS, HR-TEM, EELS, FE-SEM, EDX, TGA, BET surface area analysis, FT-IR spectroscopy, and Raman spectroscopy. The Pd content of the final system has been quantified using ICP-OES. Its applications have been explored in Suzuki-Miyaura C-C cross coupling and C-O cross coupling reactions. Hot filtration experiments corroborate the heterogeneous nature of the catalysis. It is recyclable for up to five reaction cycles in Suzuki-Miyaura and C-O cross coupling with marginal loss in performance. It also catalyzes the reactions of chloroarenes such as chlorobenzene, 4-chloroaniline, 1-chloro-4-nitrobenzene, 4-chloroacetophenone, 4-chlorobenzophenone for Suzuki coupling, and 1-chloro-4-nitrobenzene, 4-chlorobenzonitrile, chlorobenzene, and 4-chlorotoluene for C-O coupling. P-XRD, FE-SEM, and EDX study reveals that the catalytic system retains its structural originality and functionality after recycling.

Supramolecular Pd@methioine-EDTA-chitosan nanocomposite: an effective and recyclable bio-based and eco-friendly catalyst for the green Heck cross-coupling reaction under mild conditions

Supramolecular palladium(ii) supported on modified chitosan by dl-methionine using an ethylenediaminetetraacetic acid linker (Pd@MET-EDTA-CS) was designed and prepared through a simple procedure. The structure of this novel supramolecular nanocomposite was characterized by different spectroscopic, microscopic and analytical techniques including FTIR, EDX, XRD, FESEM, TGA, DRS, TEM, AA, and BET. The obtained bio-based nanomaterial was successfully investigated, as a highly efficient and green heterogeneous catalyst, in the Heck cross-coupling reaction (HCR) for the synthesis of various valuable biologically active cinnamic acid ester derivatives from the corresponding aryl halides using several acrylates. Indeed, aryl halides containing I or Br survived very well under optimized conditions to afford the corresponding products compared to the substrates with Cl. The prepared Pd@MET-EDTA-CS nanocatalyst promoted the HCR in high to excellent yields and short reaction times with minimum Pd loading (0.0027 mol%) on its structure as well as without any leaching occurring during the process. The recovery of the catalyst was performed by simple filtration and the catalytic activity remained approximately constant after five runs for the model reaction.

Pd@l-asparagine-EDTA-chitosan: a highly effective and reusable bio-based and biodegradable catalyst for the Heck cross-coupling reaction under mild conditions

In this research, a novel supramolecular Pd(ii) catalyst supported on chitosan grafted by l-asparagine and an EDTA linker, named Pd@ASP-EDTA-CS, was prepared for the first time. The structure of the obtained multifunctional Pd@ASP-EDTA-CS nanocomposite was appropriately characterized by various spectroscopic, microscopic, and analytical techniques, including FTIR, EDX, XRD, FESEM, TGA, DRS, and BET. The Pd@ASP-EDTA-CS nanomaterial was successfully employed, as a heterogeneous catalytic system, in the Heck cross-coupling reaction (HCR) to afford various valuable biologically-active cinnamic acid derivatives in good to excellent yields. Different aryl halides containing I, Br and even Cl were used in HCR with various acrylates for the synthesis of corresponding cinnamic acid ester derivatives. The catalyst shows a variety of advantages including high catalytic activity, excellent thermal stability, easy recovery by simple filtration, more than five cycles of reusability with no significant decrease in its efficacy, biodegradability, and excellent results in the HCR using low-loaded Pd on the support. In addition, no leaching of Pd into the reaction medium and the final products was observed.

Titania Nanorod-Supported Mercaptoundecanoic Acid-Grafted Palladium Nanoparticles as a Highly Reusable Heterogeneous Catalyst for Substrate-Dependent Ullmann Coupling and Debromination of Aryl Bromides

Herein, by implanting palladium nanoparticles (Pd NPs) onto titanium dioxide (TiO₂) nanorods (NRs) through 11-mercaptoundecanoic acid (MUA), we devised a robust heterogeneous catalyst. The formation of Pd-MUA-TiO₂ nanocomposites (NCs) was authenticated using Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, Brunauer-Emmett-Teller analysis, atomic absorption spectroscopy, and X-ray photoelectron spectroscopy techniques. Pd NPs were synthesized directly onto TiO₂ nanorods without the MUA support for comparative studies. As a means of evaluating the endurance and competency of Pd-MUA-TiO₂ NCs compared to their counterpart (Pd-TiO₂ NCs), both were used as the heterogeneous catalyst for Ullmann coupling of a wide variety of aryl bromides. When Pd-MUA-TiO₂ NCs were used, the reaction produced high yields of homocoupled products (54-88%), whereas the yield was only 76% when Pd-TiO₂ NCs were used. Moreover, Pd-MUA-TiO₂ NCs impressed with their outstanding reusability property, allowing over 14 reaction cycles without losing efficiency. On the flip side, just after seven reaction cycles, the productivity of Pd-TiO₂ NCs dropped around 50%. Presumably, the strong affinity of Pd for the thiol groups of MUA allowed for the substantial control of leaching out of Pd NPs during the reaction. Nonetheless, another crucial feature of the catalyst is that the di-debromination reaction took place with an excellent yield of 68-84% from di-aryl bromides with long alkyl chains instead of macrocyclic or dimerized products. It is worth mentioning that AAS data confirmed that only 0.30 mol % catalyst loading was sufficient to activate a broad substrate scope with large functional group tolerance.

Single source precursor route for the first graphene oxide-Pd6P nanocomposite: application in electrochemical hydrogen evolution reaction

For the first time, Pd₆P has been synthesised using a simple, straightforward and one-pot method i.e., thermolysis of a Pd(II) complex of a bidentate (P, N) organophosphorus ligand (anthracene-9-yl-CHN-CH₂CH₂-PPh₂). The electrocatalyst (obtained after grafting nanospheres of Pd₆P over layers of graphene oxide) shows high activity in electrochemical hydrogen evolution reactions (HER) with an overpotential of 133 mV to drive 10 mA cm^-2 of cathodic current density. The GO-Pd₆P nanocomposite is robust and effective for a continuous HER run for up to 16 hours.

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications

Palladium (Pd)-based nanomaterials have been identified as potential candidates for various types of electrocatalytic reaction, but most of them typically exhibit unsatisfactory performances. Recently, extensive theoretical and experimental studies have demonstrated that the interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic atoms (H, B, C, N, P, S) has a significant impact on their electronic structure and thus leads to the rapid development of one kind of promising catalyst for various electrochemical reactions. Considering the remarkable progress in this area, we highlight the most recent progress regarding the innovative synthesis and advanced characterization methods of nonmetallic atom-doped Pd-based nanomaterials and provide insights into their electrochemical applications. What's more, the unique structure- and component-dependent electrochemical performance and the underlying mechanisms are also discussed. Furthermore, a brief conclusion about the recent progress achieved in this field as well as future perspectives and challenges are provided.

Organoselenium ligands for heterogeneous and nanocatalytic systems: development and applications

Organoselenium ligands have attracted great attention among researchers during the past two decades. Various homogeneous, heterogeneous and nanocatalytic systems have been designed using such ligands. Although reports on selenium ligated homogeneous catalysts are quite high in number, significant work has also been done on the development of heterogeneous and nanocatalytic systems using organoselenium ligands. A review article, focusing on the utility of organoselenium compounds in the development of catalytic systems, was published in 2012 (A. Kumar, G. K. Rao, F. Saleem and A. K. Singh, Dalton Trans., 2012, 41, 11949). Moreover, it mainly covered the homogeneous catalysts. There are no review articles in the literature on heterogeneous and nanocatalytic systems designed using organoselenium compounds and their applications. Hence, this perspective aims to cover the developments pertaining to the synthetic aspects of such catalytic systems (using organoselenium compounds) and their applications in catalysis of a variety of chemical transformations. Salient features and advantages of organoselenium compounds have also been highlighted to justify the rationale behind their use in catalyst development. Their performance in various chemical transformations [viz. Suzuki-Miyaura coupling, Heck coupling, Sonogashira coupling, O-arylation of phenol, transfer hydrogenation of aldehydes and ketones, aldehyde-alkyne-amine (A3) coupling, hydration of nitriles, conversion of aldehydes to amides, cross-dehydrogenative coupling (CDC), photodegradation of substrates (formic acid, methylene blue), reduction of nitrophenols, electrolysis (hydrogen evolution reaction and oxygen reduction reactions), organocatalysis and dye sensitized solar cells] and relevant aspects of catalytic processes (such as recyclability, substrate scope and green aspects) have been critically analyzed. Future perspectives have also been discussed.

Organosulfur, organoselenium, and organotellurium ligands in the development of palladium, nickel, and copper-based catalytic systems for Heck coupling

Organosulfur, organoselenium, and organotellurium ligands in designing Pd, Ni, and Cu-based homogeneous, heterogeneous, and nanocatalytic systems for Heck coupling.

Organochalcogen ligands in catalysis of oxidation of alcohols and transfer hydrogenation

Organochalcogen compounds have been used as the building blocks for the development of a variety of catalysts that have been studied comprehensively during the last two decades for several chemical transformations. Transfer hydrogenation (reduction of carbonyl compounds to alcohols) and oxidation of alcohols (conversion of alcohols to their respective ketones and aldehydes) are also among such chemical transformations. Some compilations are available in the literature on the development of catalysts, based on organochalcogen ligands, and their applications in Heck reaction, Suzuki reaction, and other related aspects. Some review articles have also been published on different aspects of oxidation of alcohols and transfer hydrogenation. However, no such article is available in the literature on the syntheses and use of organochalcogen ligated catalysts for these two reactions. In this perspective, a survey of developments pertaining to the synthetic aspects of such organochalcogen (S/Se/Te) based catalysts for the two reactions has been made. In addition to covering the syntheses of chalcogen ligands, their metal complexes and nanoparticles (NPs), emphasis has also been placed on the efficient conversion of different substrates during catalytic reactions, diversity in catalytic potential and mechanistic aspects of catalysis. It also includes the analysis of comparison (in terms of efficiency) between this unique class of catalysts and efficient catalysts without a chalcogen donor. The future scope of this area has also been highlighted.

Catalysis with magnetically retrievable and recyclable nanoparticles layered with Pd(0) for C–C/C–O coupling in water

Nanoparticles layered with palladium(0) were prepared from nano-sized magnetic Fe₃O₄ by coating it with silica and then reacting sequentially with phenylselenyl chloride under an N₂ atmosphere and palladium(ii) chloride in water. The resulting Fe₃O₄@SiO₂@SePh@Pd(0) NPs are magnetically retrievable and the first example of NPs in which the outermost layer of Pd(0) is mainly held by selenium. The weight percentage of Pd in the NPs was found to be 1.96 by ICP-AES. The NPs were authenticated via TEM, SEM-EDX, XPS, and powder XRD and found to be efficient as catalysts for the C–O and C–C (Suzuki–Miyaura) coupling reactions of ArBr/Cl in water. The oxidation state of Pd in the NPs having size distribution from ∼12 to 18 nm was inferred as zero by XPS. They can be recycled more than seven times. The main features of the proposed protocols are their mild reaction conditions, simplicity, and efficiency as the catalyst can be separated easily from the reaction mixture by an external magnet and reused for a new reaction cycle. The optimum loading (in mol% of Pd) was found to be 0.1–1.0 and 0.01–1.0 for O-arylation and Suzuki–Miyaura coupling, respectively. For ArCl, the required amount of NPs was more as compared to that needed for ArBr. The nature of catalysis is largely heterogeneous.

Fe₃O₄@SiO₂@SePh@Pd(0) (Pd, 1.96%) as the first example of NPs having a Pd(0) layer held by selenium can execute C–C/C–O coupling in 2–6 h (80 °C).

Palladium decorated on a new dendritic complex with nitrogen ligation grafted to graphene oxide: fabrication, characterization, and catalytic application

Immobilized Pd nanoparticles on a new ligand, namely, tris(pentaethylene-pentamine)triazine supported on graphene oxide (Pdnp-TPEPTA(L)-GO) was introduced as a novel and robust heterogeneous catalyst for use in C-C bond formation reaction. The Pdnp-TPEPTA(L)-GO catalyst was synthesized by complexation of Pd with TPEPTA as a ligand with high N-ligation sites that were supported on graphene oxide through 3-chloropropyltrimethoxysilane. The prepared catalyst was characterized using some microscopic and spectroscopic techniques. The TPEPTA(L)-GO substrate is a 2D heterogeneous catalyst with a high specific surface area and a large amount of N-ligation sites. The Pdnp-TPEPTA(L)-GO catalyst used in the C-C bond formation reaction between aryl or heteroaryl and phenylboronic acid derivatives was applied towards the synthesis of biaryl units in high isolated yields. Notably, a series of competing experiments were performed to establish the selectivity trends of the presented method. Also, this catalyst system was reusable at least six times without a significant decrease in its catalytic activity.

Solvent-tailored Pd3P0.95 nano catalyst for amide-nitrile inter-conversion, the hydration of nitriles and transfer hydrogenation of the C[double bond, length as m-dash]O bond

For the first time, a one pot thermolysis of [Pd(PPh3)2Cl2] prepared by reacting Ph3P with PdCl2 in a 2 : 1 molar ratio in MeOH at 280 °C in a trioctylphosphine (TOP) and oleylamine(OA)-octadecane(ODE) mixture (1 : 1) was used to prepare quantum dots (QDs; size ∼2-3 nm) and nanoparticles (NPs; size ∼13-14 nm), respectively, of composition Pd3P0.95. TEM, SEM-EDX, powder-XRD and XPS (for QDs only) were used to authenticate the two nanophases. 31P{1H}NMR experiments performed to monitor the progress of thermolysis reactions revealed that the phosphorus present in the Pd3P0.95 QDs had come from TOP, whereas in Pd3P0.95 NPs, its source is triphenylphosphine. The nature of the solvent did not affect the chemical composition of the nano-phase but controlled its size. Probably, solvent dependent, unique, single source precursors (SSPs) of palladium were generated in situ, and controlled the size. The catalytic activity of both Pd3P0.95 QDs and NPs was explored. The QDs were found to be efficient as a catalyst for the amide-nitrile interconversion at room temperature (yield up to 92% in 4 h), hydration of nitriles and transfer hydrogenation (TH) of carbonyl compounds with yields up to 96% in 3-4 h. The yields and reaction rates of amide-nitrile inter-conversion and TH when catalyzed by Pd3P0.95 QDs were found to be higher compared to the ones observed with the Pd/C catalyst. The binding energy of Pd(3d) in the X-ray photoelectron spectrum (XPS) of Pd3P0.95 indicated an electron transfer from the metal to phosphorus, resulting in electron deficient palladium, which facilitates the coordination of a substrate to Pd and drives the reaction. The reusability of Pd3P0.95 QDs for the interconversion was found to be up to 4-times, while for the transfer hydrogenation of carbonyl compounds it was up to 6-times, but with a diminished yield. Pd3P0.95 NPs were found to be less active (yield up to 36% in optimized reaction conditions) in comparison to Pd3P0.95 QDs. The mercury poisoning test suggested that the catalysis predominantly proceeded heterogeneously on the surface of the QDs. The PXRD and XPS results did not suggest a significant variation in the phase of QDs after the third catalytic cycle. The bleeding of Pd during catalysis (determined by flame AAS) and the agglomeration of QDs as supported by the SEM-EDX and TEM results are probably responsible for the reduction in the catalytic activity of QDs after reusing three times.

Palladium Nanoparticles Supported on Graphene Oxide as Catalysts for the Synthesis of Diarylketones

Three palladium catalysts supported on graphene oxide (GO) and on its composite with TiO2 (GO-TiO2) were prepared and characterized. The presence of Pd NPs of different diameters (4–89 nm) and size distributions was evidenced by TEM measurements. GO-supported palladium efficiently catalysed the carbonylative coupling of iodobenzenes with aryl boronic acids forming relevant diarylketones at 1 atm CO. The highest activity and recyclability were obtained for Pd/GO-TiO2. The emission behaviour of Pd/GO and Pd/GO-TiO2 catalysts indicated structural changes occurring during the catalytic reaction.

Palladium Nanoparticle-Enabled Ultrathick Tribofilm with Unique Composition

There is a consensus that savings of 1.0-1.4% of a country's gross domestic product may be achieved through lubrication R&D. Recent studies have shown great potential for using surface-functionalized nanoparticles (NPs) as lubricant additives to enhance lubricating performance. NPs were reported with ability of producing a low-friction antiwear tribofilm, usually 20-200 nm in thickness, on the contact surface. In contrast, this study reports an unexpected 10 times thicker (2-3 μm) tribofilm formed by dodecanethiol-modified palladium NPs (core size: 2-4 nm) in boundary lubrication of a steel-cast iron contact. Adding 0.5-1.0 wt % such NPs to a lubricating oil resulted in significant reductions in friction and wear by up to 40 and 97%, respectively. Further investigation suggested that the PdNP core primarily was responsible for the improvement in both friction and wear, whereas the thiolate ligand only contributed to the wear protection but had little impact on the friction behavior. In addition, unlike most previously reported tribofilms that contain a substantial amount of metal oxides, this PdNP-induced tribofilm is clearly dominated by Pd/S compounds, as revealed by nanostructural examination and chemical analysis. Such a ultrathick tribofilm with unique composition is believed to be responsible for the superior lubricating behavior.

Monodisperse Palladium Sulfide as Efficient Electrocatalyst for Oxygen Reduction Reaction

In this study, we report a colloidal synthesis of palladium sulfides (including Pd₁₆S₇, Pd₄S, and PdS) via a facile one-pot hot-solution synthetic route and their promising application as electrocatalyst for the oxygen reduction reaction (ORR). Among the different palladium sulfides tested, monodisperse Pd₄S nanoparticles exhibit the best electrocatalytic activity toward ORR in alkaline medium, with the half-wave potential ca. 47 mV more positive than that of the state-of-the-art Pt/C catalyst. Density functional theory calculations indicate the existence of oxygen absorption sites in Pd₄S surface result in optimized oxygen-binding ability for the four-electron oxygen reduction.

Hydrotalcite-supported palladium nanoparticles as catalysts for the hydroarylation of carbon–carbon multiple bonds

Palladium nanoparticles supported on hydrotalcites catalyze the hydroarylation reaction of carbon–carbon multiple bonds.

GO–Cu7S4 catalyzed ortho-aminomethylation of phenol derivatives with N,N-dimethylbenzylamines: site-selective oxidative CDC

An efficient ortho-selective C–H aminomethylation of phenols and naphthols with N,N-dimethylbenzylamines using a GO–Cu₇S₄ nanocatalyst is reported.

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation.

Owing to their unique properties, hollow metal nanocrystals demonstrate greater catalytic promise than their solid counterparts. Here the authors produce hollow and inflated palladium nanocrystals with thin shells via a repeated Kirkendall cavitation process, and demonstrate their activity for formic acid oxidation.

Trinuclear complexes of palladium(ii) with chalcogenated N-heterocyclic carbenes: catalysis of selective nitrile–primary amide interconversion and Sonogashira coupling

The interconversion of nitriles to amides and Sonogashira coupling were catalyzed with Pd(ii) complexes (0.5–2 mol%).

Structure and energetics of graphene oxide isomers: ab initio thermodynamic analysis

Graphene oxide (GO) holds significant promise for electronic devices and nanocomposite materials. A number of models were proposed for the GO structure, combining carboxyl, hydroxyl, carbonyl and epoxide groups at different locations. The complexity and variety of GO isomers, whose thermodynamic stability and formation kinetics depend on the applied conditions, make determination of the GO structure with atomistic precision challenging. We report high level theoretical investigation of multiple molecular configurations, which are anticipated in GO. We conclude that all oxygen containing groups at the GO surface are thermodynamically permitted, whereas the 'edge' positions are systematically more favorable than the 'center' and 'side' positions. We discuss a potentially novel type of chemical bond or bonding reinforcement in GO, which consists of a covalent bond and a strong electrostatic contribution from a polarized graphene plane. We observe and analyze significant modifications of the graphene geometry and electronic structure upon oxidation. The reported thermodynamic data guide experiments aimed at deciphering the GO chemical composition and structure, and form the basis for predicting GO properties required for nano-technological applications.

Megranate-like nanoreactor with multiple cores and an acidic mesoporous shell for a cascade reaction

Megranate-like nanoparticles possess a unique structure that is composed of multiple cores and shells, which is different from simple yolk-shell nanoparticles. Megranate-like nanoparticles can combine the properties of each component and be used as nanoreactors. This study describes the preparation of bifunctional megranate-like nanoreactors, consisting of multiple metal cores and thiol modified mesoporous SiO2 shells. Different metal nanoparticles (Pd, Pt, Au) can be incorporated into the structure as cores, and the thiol group in the shells can be oxidized to acidic -SO3H. The megranate-like nanoparticles show good bifunctional catalytic properties and recyclability in a cascade catalytic reaction for the desired benzimidazole derivative. Moreover, the individual components of the megranate-like nanoparticles also show good catalytic activities in the hydrogenation reduction of nitro-aromatics and the deprotection reaction of benzaldehyde dimethyl acetal.

Palladium(ii)-1-phenylthio-2-arylchalcogenoethane complexes: palladium phosphide nano-peanut and ribbon formation controlled by chalcogen and Suzuki coupling activation

Suzuki coupling (cocktail pathway) by complexes of S/Se-ligand/PdP₂ NPs prepared from Pd-complexes in TOP (temperature ≥ 280 °C) is efficient.